Radar level gauges (RLGs) are suitably used for making non-contact measurements of the level of products such as process fluids, granular compounds and other materials contained in a tank. An example of such a radar level gauge can include a transceiver for transmitting and receiving microwaves, an antenna arranged to direct microwaves towards the surface and to return microwaves reflected by the surface to the transceiver, and processing circuitry adapted to determine the level based on a relation between microwaves transmitted and received by the transceiver.
In some applications, the antenna is mounted inside the tank and is exposed to the tank contents and environment. However, in some cases it is desirable to insulate the RLG and its antenna from the tank contents. For example, it may be advantageous to provide a smooth surface towards the tank interior, thus avoiding contamination of hidden compartments present in a complex structure such as an antenna.
A challenge with such a design, however, is that it requires an opening in the tank corresponding to the antenna cross section, typically in the range of 50-400 mm. The size of the opening can be a mechanical problem, if the opening has to withstand high pressure, especially if the temperature is high enough to cause an insulating material covering at least a part of the antenna opening to lose some of its strength.
Several solutions are known to address these problems. According to one of these solutions, a radome (also referred to as window seal or radar window) is arranged below the antenna. An example of such a radome is given in U.S. Pat. No. 6,325,391 to Smith et al., where a window seal is arranged at an interface between a flange of the tank and a standoff pipe. A radar gauge is mounted on the standoff pipe, so that the antenna extends into the pipe, with its opening just above the window seal. The window seal is formed as an inverted cone made of PTFE, and is thick enough to withstand the pressure of the tank.
However, in some applications the pressure in the tank exceeds that which the PTFE window seal disclosed in U.S. Pat. No. 6,325,391 can withstand, in particular when the temperature is high. There is thus a need for an improved solution, which can withstand even higher pressure.